Products such as concrete structures and plasters, which are bound with cement and corresponding hydraulic binders, are very common and well studied. The problems associated with them are familiar, too. The problems of the production process are related, among other things, to the water which is left over when the chemical water, required by the reaction of the hydraulic binder, and the gel water have been consumed. This residual water affects the workability of the product. Plasticizers have been developed to reduce the quantity of this residual water, but these are attached to the reactive spots of the cement particles and compete with the ions generated in the hydration reaction. The plasticizers are generally organic polymers and it is not desirable to use large quantities of them in concrete.
Another major drawback is the calcium hydroxide, Ca(OH)2, which is generated during the hydration process, the amount generated being 0.29 kg per each kilogram of cement. Some of the Ca(OH)2 crystals are hexagonal plate packs, the structure of which is weak. To overcome this problem, pozzolanic materials, such as silica (SiO2), are employed to reduce the amount of calcium hydroxide. However, although pozzolanic materials use up calcium hydroxide during the pozzolanic reaction, they require more water in order to loosen the structure than the pozzolanic reaction consumes.
In known concrete structures, micro cracks in hardened cement still appear, which cracks are generated because of, among other things, autogenic shrinkage. To prevent these, a higher w/c ratio is needed than the chemical water and gel water require. The “w/c” ratio means the weight ratio between water and binder.
Yet another problem area is the joint zone, i.e. the transition zone between the cement paste and the aggregate, in which zone most internal cracks in the concrete occur. When fractures in concrete structures are examined, it is found that the fracture always starts in this joint zone and then extends into the plaster. It is possible to reduce the size of the transition zone by decreasing the w/c ratio, but problems occur: either the workability is reduced or more plasticizer is needed.
Yet a further problem is that with modern technology the heat treatment must be limited because the heat expansion of concrete has been in the past non-linear and the various components of the concrete have had different coefficients of heat expansion. There have been affected by the air dissolved in water, the air in gaseous state and the water in the concrete. Consequently, it has not been possible to fully exploit the shortened processing time enabled by heat treatment—according to a rule of thumb, the processing time is cut by half for each 10 degree rise in the temperature. Industrial production of construction components requires a shorter concrete hardening time, which is made possible by moving from mechanisation to automation, which cuts production costs. With cast-in-place processes, for instance, heat treatment speeds up the work, too.
The estimated useful lifespan of modern concretes is only 50 years, which demonstrates the extent of the problems. In 2005, a useful lifespan of 100-200 years was introduced into the standards. This is mainly a result of the decision to increase the protective distance of steel. However, circumstances which essentially would increase the useful life of concrete remain unresolved.